[Voxtral] Raise BadRequest for ref_audio (#7012)

Signed-off-by: Clodagh Walsh <clodaghwalsh17@gmail.com>

tests/entrypoints/openai_api/test_serving_speech.py

@@ -5133,6 +5133,7 @@ class TestTTSAsyncOffloading:
 
     def test_prepare_speech_generation_awaits_voxtral_async(self, voxtral_server, mocker: MockerFixture):
         """Voxtral path in _prepare_speech_generation should call the async wrapper."""
+        voxtral_server._adapter._encoder_loaded = mocker.AsyncMock(return_value=False)
         voxtral_server._adapter._build_prompt_async = mocker.AsyncMock(
             return_value={
                 "prompt_token_ids": [1, 2, 3],
@@ -5288,6 +5289,7 @@ class TestTTSAsyncOffloading:
         """FINAL_ONLY streaming for async_chunk=False is scoped to qwen3_tts only."""
         voxtral_server.engine_client.model_config.async_chunk = False
         mocker.patch.object(voxtral_server._get_tts_adapter(), "validate", return_value=None)
+        voxtral_server._adapter._encoder_loaded = mocker.AsyncMock(return_value=False)
         voxtral_server._adapter._build_prompt_async = mocker.AsyncMock(
             return_value={
                 "prompt_token_ids": [1, 2, 3],